The present invention relates to printing devices and more particularly to printing devices with a platen mechanism, and a method of controlling the printing device.
In printing devices, such as impact printers, the gap between the print heat and the platen is one of the key factors that determine print quality. If the gap is too large, the print head loses its proper distance for printing on the surface of a printing sheet. This results in blur and deformation of printed characters. With ink jet printers, if the gap is excessively large, an error of the ink drop arrival position caused by an error of the ejecting angle is enlarged, so that the print quality is also deteriorated. For this reason, to secure a high print quality, the platen-to-print head gap should be reduced by locating the platen as close to the print head as possible in a state that the platen is opposite the print head.
Typically, a printing sheet is transported from the sheet supply side to a printing region by a transporting mechanism, such as rollers, through a sheet transport path. The height of the inner space within the transport path is large enough to allow the printing sheet to smoothly move therethrough, thereby preventing the sheet form being jammed in the transport path. Accordingly, the printing sheet must be inserted in a narrower gap in the printing region form the large enough transport path. As a result, the leading edge of the sheet hits the platen or a member near to it, possibly causing a paper jam.
One technique to solve the problem is known. This conventional, prior art platen mechanism is illustrated in FIGS. 8 to 10. As shown, a platen 81 is disposed opposite a print head 80, which is horizontally movable. The platen 81 consists of a plate-like member having a length corresponding to a movable range of the print head 80. The platen 81 is fastened to a support member 83, which may be turned about a shaft 82. The support member 83 is mechanically coupled with a solenoid plunger 84. When driven by the plunger 84, the support member 83 and hence the platen 81 are moved to and from the print head 80.
In a print mode of the printer, the platen 81 is moved toward the print head 80 and its end face is positioned in a sheet transport path 85. Accordingly, a reduced gap G is set up between the platen 81 and the print head 80 (FIG. 9).
In a sheet insertion mode of the printer in which a sheet is inserted into a printing region (including a portion of the transport path 85 located between the platen 81 and the print head 80), the platen 81 is retracted from the transport path 85, and hence its end face is moved apart from the print head 80 and the gap G is increased. Under this condition, the printing sheet P may enter the printing location smoothly or without any interruption.
This conventional platen mechanism suffers from problems. In operation, the entirety of the combination of the platen 81 and the support member 83 is pivoted by the plunger 84. To keep the gap G constant when the printer is in the print mode, exactness is required for the dimensions of the related parts. In addition, a large space if required between the opening of the transport path 85 and the platen 81. The moving sheet is frequently caught by the large space, resulting in a paper jam. Finally, the platen 81 needs to be stable when it receives an impact from the print head 80. To this end, the output power of the plunger 84 must be large.
Another prior art platen mechanism is disclosed in JP-A-8-11365. In the prior art, the gap between the print head and the platen is adjusted such that the gap is increased in the sheet insertion mode, and decreased in the print mode. An eccentrically mounted cylindrical platen is rotated to adjust the gap.
In a design where the eccentrically mounted cylindrical platen protrudes beyond the guide wall defining the transport path, the radius of the movement of the cylindrical platen is larger than that of the platen per se, so that a large gap is created between the platen and the guide wall. This large gap can easily catch the leading edge of the printing sheet, causing a paper jam.
There are cases where the leading edge of the printing sheet merely hits the guide wall portion defining the gap, not causing a paper jam. In this case, the transporting speed of the printing sheet is varied. Therefore, when the printer engages in reading information on the sheet or Magnetic Ink Character Recognition (MICR) characters, the speed variation appears as read errors.
The present invention aims at providing a platen mechanism in use with a printing device which, in the print mode, keeps the gap in the printing region constant, and, in the sheet insertion mode, allows a sheet to smoothly pass the gap without any adverse effect on the sheet transporting speed.
A platen mechanism, disposed opposite a print head, is provided for supporting a recording medium. The platen mechanism includes a guide wall for guiding the recording medium to the print head, a platen which is rotatable about a shaft substantially parallel to the guide wall and has a support portion and a guide portion. When the rotating platen is stopped at a first position where the support portion is opposite the print head, the support portion projects from the guide wall to support the recording medium to the print head. When the rotating platen is stopped at a second position where the guide portion is opposite the print head, the guide portion is substantially continuous to the guide wall to smoothly guide the recording medium being transported. A platen drive mechanism rotates the platen so that the support portion or the guide portion selectively confronts the print head.
With such a mechanical arrangement, when the printing device is in a print mode, the platen is turned to confront the support portion of the platen with the print head, and it serves as a platen. In this case, a force applied from the print head (such as a dot matrix or other impact print head) to the platen may be received by the rotary shaft of the platen. In accordance with the present invention, the platen is immovable and stable printing is ensured. When the printing device is in a medium transporting mode, the recording medium passes through a printing region (including a portion of the transport path located between the platen and the print head), the platen is turned to confront the guide portion of the platen with the print head. In this case, the guide portion is substantially continuous to the guide wall, so that the recording medium being transported is smoothly guided. The result is to prevent a variation of the transporting speed of the recording medium.
The platen mechanism may comprise an urging member which urges the recording medium apart from the print head and supports the recording medium at a predetermined distance from the print head in cooperation with the support portion of the platen. In this case, the guide portion of the platen is preferably positioned apart from the urging member when the guide portion is opposite the print head. With the additional feature, in the print mode, the distance between the recording medium and the print head can be kept constant. Therefore, in the ink jet printer, for example, print quality deterioration caused by a print position variation that is due to the ejecting angle error of the ink drop is eliminated. In the medium transporting mode, the urging member is separated from the guide portion of the platen to open and to provide a large sheet transporting path. Therefore, the recording medium advances without any collision and hence is free from the transporting speed variation caused by the collision of the recording medium. In the platen mechanism, when the guide portion is opposite the print head, the guide portion of the platen matches the ends of the guide wall. The guide portion of the platen is disposed with respect to the guide walls. A minimum gap is present between each end of the guide portion and the end of the corresponding guide wall. The minimum gap allows the platen to move angularly. With this unique mechanical feature, there is less chance that the recording medium hits the guide portion of the platen, and hence the medium transportation speed is not varied.
In the platen mechanism, the platen includes an introducing surface for introducing the recording medium from the end of the guide wall to the support portion when the support portion confronts the print head. Use of the introducing surface, allows the recording medium to smoothly pass through the printing region in a print ready state (the support portion of the platen confronts the print head), where a variation of the transporting speed of the recording medium offers no problem.
In one embodiment, a printing device can comprise a pair of rollers for transporting the recording medium when the print head prints on the recording medium, a roller-pair drive mechanism for moving the paired rollers to and from each other, and an interlinking mechanism for mechanically linking the platen drive mechanism to the roller-pair drive mechanism such that when the platen drive mechanism moves the platen to the first position, the roller-pair drive mechanism moves the paired rollers to each other to bring them into contact with each other, and when the platen drive mechanism moves the platen to the second position, the roller-pair drive mechanism moves the paired rollers apart from each other.
The printing device reliably transports the recording medium in a print mode, and retracts the platen while at the same time moves the paired rollers out of the transport path. The printing device thus arranged realizes quick sheet processing, easy control and reduction of the number of required drive sources.
In an embodiment of the printing device, the platen drive mechanism can include a drive shaft, a clutch mechanism for intermittently transmitting a drive force from the drive shaft to the platen, and a trigger mechanism for setting up a coupling state of the clutch mechanism at a predetermined timing, and a decoupling state of the clutch mechanism in accordance with a predetermined quantity of rotation of the platen. These technical features enable the drive shaft to be used together with other mechanisms, such as a medium transporting mechanism. This results in reduction of the number of required drive sources, and hence reduction of the device size. The transportation of the recording medium and the securing of the transport path and the preparation for printing may be concurrently carried out, and therefore the sheet processing is quick.
In another embodiment of the invention a printing device includes a print head for printing on a recording medium, a detector for reading information on the recording medium, a sheet transporting mechanism for transporting the recording medium in front of the print head and the detector, and a guide wall for guiding the recording medium being transported by the transporting mechanism to the print head. A platen is disposed opposite the print head and movable to a first position where the support portion thereof is projected from the guide wall to support the recording medium to the print head, and to a second position where the guide portion thereof is substantially continuous to the guide wall to smoothly guide the recording medium being transported. A platen drive mechanism can drive the platen to move selectively to the first or second position. A controller controls the platen drive mechanism to move to the first position when printing on the recording medium by the print head, and to move to the second position when reading information by the detector.
The printing device thus constructed exactly sets the platen to predetermined positions in a print mode and a sheet transporting mode.
In such a printing device, the platen disposed confronting the print head is rotatable about a shaft substantially parallel to the guide walls and includes a support portion and a guide portion. When the rotating platen is at a first position where the support portion confronts the print head, the support portion is projected from the guide wall to support the recording medium to the print head. When the rotating platen is at a second position where the guide portion confronts the print head, the guide portion is substantially continuous to the guide wall to smoothly guide the recording medium being transported therethrough.
The combination of the controller and the platen completely removes a variation of the sheet transporting speed when information is read out of the recording medium.
The detector may be a magnetic head for reading information from the recording medium. The operation of reading the MICR characters is sensitive to the speed variation. Use of the magnetic head is very useful, in this respect. Other types of readers, such as optical character or magnetic stripe readers, can also be used.
The printing device may further comprise a command interpreter for interpreting a control command received from a host device connecting to the printing device, wherein the controller moves the platen to the second position when the command interpreter interprets a predetermined command to require the detector to read information. This arrangement secures a sheet transport path in preparation for the operation of reading information from the recording medium.
The invention further provides a method of controlling a printing device comprising the steps of: transporting a recording medium on and along guide walls, supporting the recording medium to a print head in a state that a platen is projected to a support position, printing on the recording medium by the print head, guiding the recording medium in a state that the platen is retracted to a guide position, reading information from the recording medium transported in the first step by use of a detector.
In this case, the control method may further comprise a step of interpreting a predetermined control command received from a host device connecting to the printing device and to require the reading of information on the recording medium, wherein the step of reading information from the recording medium is executed when the control command is interpreted in the step of interpreting the control command.
Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings, and the claims.